This invention relates to a method for the preparation of 2-(trifluoromethyl)phenyl carbamic fluorides. The compounds prepared are useful as chemical intermediates for the further preparation of aromatic amines, especially 2-(trifluoromethyl)-benzenamine.
The 2-(trifluoromethyl)-benzamines, also known as o-trifluoromethyl anilines or o-aminobenzotrifluorides, are a known class of compounds having commercial utility as chemical intermediates, particularly as intermediates for the preparation of various dyestuff and pesticides. For example 2-amino-5-chlorobenzotrifluoride is employed commercially as a dye intermediate and is designated as C.I. Azoic Diazo Component 17, according to Colour Index, Chemical No. 37055, Vol. 1-4, 2nd ed. 1956, Suppl. 1963 published by the Society of Dyers and Colourists (U.K.) and The American Association of Textile Chemists and Colorists (U.S.).
The trifluoromethylanilines are disclosed in U.S. Pat. No. 4,243,819, to Henrick et al., as intermediates in the synthesis of amino acid esters having pesticidal properties. Thus, for example, the reference teaches the preparation of the m-phenoxybenzyl ester of N-(2-trifluoromethylphenyl)valine by reaction of 2-trifluoromethylaniline with m-phenoxybenzyl .alpha.-bromoisovalerate.
U.S. Pat. No. 4,316,988 to Clinton discloses the use of trifluoromethyl-substituted anilines as intermediates in the synthesis of various diphenylamine products useful as rodenticides, insecticides, and arachnicides.
The utility of the o-aminobenzotrifluorides as chemical intermediates has led to the investigation and development of various methods for the preparation of these compounds.
Forbes et al., Tetrahedron, Vol. 8, 67-72 (1960) prepared o-aminobenzotrifluoride by hydrogenation of 2-nitrotrifluoromethylbenzene at elevated temperatures and pressure in the presence of a Raney nickel catalyst.
German Offenlegungschrift D.E. No. 3,017,542 to Klauke et al. discloses the preparation of o-aminobenzotrifluoride by hydrogenation-hydrogenolysis of 2-amino-5-chlorobenzotrifluoride.
McBee et al., J. Am. Chem. Soc. 73, 3932-34 (1951) disclose the preparation of 4-bromo-2-(trifluoromethyl)-aniline by nitration of 3-bromo-(trifluoromethyl)-benzene followed by reduction of the resultant 2-nitro-5-bromo(trifluoromethyl)-benzene.
Although methods for the preparation of 2-(trifluoromethyl)-benzenamine are known from the prior art, it will be appreciated that the development of improved and more economical processes is desirable. The present invention provides a convenient and economical method for the preparation of 2-(trifluoromethyl)phenyl carbamic fluorides, which, in turn, have been found particularly useful as an intermediate for the preparation of aminobenzotrifluorides and hydrofluoride salts thereof.
The preparation of phenyl carbamic acid fluorides from aromatic isocyanates is known. Buckley et al. J. Chem. Soc. 864 (1945) disclose the preparation of phenyl carbamic fluorides by reaction of HF with various isocyanates. However, the reference provides no teaching relative to the preparation of phenyl carbamic fluorides bearing a fluoroalkyl side chain.
British Pat. No. 955,898 (1964) to Farbenfabriken Bayer Aktiengesellschaft discloses the reaction of anhydrous hydrogen fluoride with chloromethylphenyl isocyanates to produce the corresponding fluoromethylphenyl carbamic fluoride, or, upon subsequent heating, the corresponding isocyanate. The British patent further discloses the reaction of hydrogen fluoride with 2-trichloromethyl-4-chlorophenyl isocyanate in chlorobenzene to prepare 2-trifluoromethyl-4-chlorophenyl carbamic acid fluoride. Subsequently, Klauke, Angew. Chem. Interat. Ed. Vol. 5, No. 9, 848, (1966), in contrast to the teachings of British Pat. No. 955,898, stated that when o-trichloromethylphenyl isocyanate undergoes Cl--F exchange in anhydrous hydrogen fluoride, isomerization occurs simultaneously and o-N-(trifluoromethyll)aminobenzoyl fluoride can be isolated, thus implying that o-trifluoromethyl phenyl carbamic fluoride is not produced.
In U.S. Pat. No. 3,829,460 to Buttner and Klauke, assigned to Bayer Aktiengesellschaft, reference is made to the 1966 article and to earlier contradictory teachings and it is disclosed that when hydrogen fluoride is reacted with a trichloromethylphenyl isocyanate wherein the trichloromethyl group is in the 2-position to the isocyanate group, it is only possible to obtain the isomer, 2-N-trifluoromethylaminobenzoyl fluorides.
British Pat. No. 1,164,223 to Klauke et al. teaches the hydrolysis of trifluoromethylphenyl isocyanates with 90-100 percent sulfuric acid to produce the corresponding trifluoromethyl benzenamine (or amine.sulfate) with carbon dioxide as the only byproduct.
The hydrolysis of an NCO group with acid agents, such as concentrated hydrochloric acid or sulphamic acid is known (Houben-Weyl, Methoden der org. Chemie, 4th Edition, Vol. 11/1, page 953).